The understanding of the swelling phenomenon of montmorillonite is essential to predict the physical and chemical behavior of clay-based barriers in radioactive waste disposal systems. This study investigated the key factors controlling crystalline swelling behavior of montmorillonite with different interlayer counter-ions by molecular dynamics (MD) simulations. On the basis of the comparisons between MD simulated and experimental results, the water content in the interlayer in five homoionic (Na-, K-, Cs-, Ca- and Sr-) montmorillonite was strongly correlated to the hydration number of each counter-ion. The analyses for these results offer insights that the hydration number is controlled by the hydration free energy, the volume and the distribution of each interlayer counter-ion. The systematic MD simulations with virtually variable parameters clarified that the hydration free energy and the charge of interlayer counter-ions compete as influencing factors, and the control the formation rate of an outer-sphere complex of each counter-ion. The empirical relationships between these key factors will allow essential insights into predicting the swelling behavior of montmorillonite with different interlayer counter-ions.

  For selecting the HLW geological disposal site, it is necessary to evaluate the groundwater chemical and physical stability and inflowing of deep-seated fluids which is idiomatically classified as slab-derived fluid, fossil seawater, and oil-field brine. Among them, the slab-derived fluid is found in some areas in Japan, they often have high temperature, high CO₂ gas, and low pH characters. The features may have adverse impact for geological disposal systems, hence, consolidation of the methodology for finding the fluid is required. However, according to previous studies, the terminology of deep-seated fluids is somewhat ambiguous, and chemical characteristics of each deep-seated fluids does not well organized. In this study, the authors classify the terminology of deep-seated fluids by the formation mechanisms and the reduction of common chemical features of deep-seated fluids are conducted based on the own field surveys over 30 springs and previous studies. The origin of the deep-seated fluids could be classified by dissolved chemicals such as concentration of Cl, He isotope ratio and stable isotope ratio combined with concentrations of other dissolved ions and gases. Dissolved chemicals could be also useful tracer to find out the area of influence of the deep-seated fluid.

  Swelling pressure tests have been conducted to understand the swelling properties of bentonite which is planned to be used as a buffer material in repositories for the geological disposal of radioactive waste. It has been reported that the swelling pressure obtained by swelling pressure test increases monotonically or temporarily decreases after increasing with the passage of the time depending on the initial water content. In this study, swelling pressure tests were conducted under several different initial water content conditions, and the change in wet density distribution inside the specimen during the swelling pressure tests was observed by X-ray CT measurement. It was confirmed that the presence or absence of collapse behavior and its magnitude affect the shape of the change in swelling pressure. When the collapse deformation was large, the amount of temporary decrease in swelling pressure was large. On the other hand, swelling pressure increased monotonically when there was no deformation due to collapse.

  Since fractures in the crystalline rock such as granite can act as the pathways for groundwater flow and mass transport, understanding of the fracture distribution is an important subject for the disposal of high-level nuclear waste. Fracture data obtained through the borehole investigation from the ground surface contains errors due to the crossing angle between boreholes and fractures. Therefore, I studied method of borehole investigation that can effectively characterize the fracture distribution based on fracture information obtained from geological investigation of ventilation shaft wall at the Mizunami Underground Research Laboratory. As a result, it was found that inclined lines on shaft wall captured larger number of fractures, especially high-angle fractures, than vertical ones. This indicates that inclined boreholes are preferable for efficient capture of the fractures. However, since the fracture orientation varies with depth, it is difficult to uniquely determine the drilling direction. On the other hand, predominant orientation of fractures at surface exposure, which locates close to Mizunami Underground Research Laboratory, is well concordant with average of strike and dip of fractures observed at ventilation shaft, which implies that the drilling direction should be determined based on the result of surface fracture mapping.

  This paper reviews tectonic uplift and surface erosion within the scope of intermediate depth disposal in Japan. With regard to the endorheic area in the middle reach of rivers, sediment supply increases during glacial periods while the riverbed is eroded during interglacial periods. The erosion rate is almost equal to the uplift rate in the long term, if the same longitudinal river profile reappears in the same climate period beyond one glacial-interglacial cycle for about hundred thousand years in the area occurring crustal movement. However, the erosion rate is higher than uplift rate for interglacial periods. Therefore, for interglacial periods shorter than hundred thousand years which corresponds to the evaluation period of intermediate depth disposal, the maximum erosion depth of the riverbed may have a large impact on the disposal site. On the other hands, in the lower reach of rivers near the coast, the erosion rate is higher than uplift rate for glacial periods with lowering of the sea level and the base level of erosion. Therefore, the maximum erosion depth may have a significant impact on the disposal site for glacial periods shorter than hundred thousand years. For evaluating tectonic uplift based on the previous studies at disposal sites, it is important to examine the applicability of the data considering their timescale.